1. Field of the Invention
The present invention relates to an improved mass flowmeter.
2. Description of the Prior Art
The conventional general mass flowmeter comprises a body block provided with a fluid passage formed therewithin, a sensor case made of resins or metals projected from said body block, a metallic capillary tube (having an outside diameter of for example about 0.6 mm and an inside diameter of for example about 0.4 mm) provided within said sensor case as a measuring passage, a pair of sensor coils wound around said capillary tube and a lead wire or a lead rod mounted on the body block for leading an electric current to said sensor coils.
However, the above described sensor case has generally attempted to isolate the sensor coil provided therewithin from the air or electro-magnetic waves, so that the sensor case, the body block and the lead wire or the lead rod and the body block have been sealed up with resinous adhesives or resinous materials.
However, according to the above described sealing-up method using resinous adhesives or resinous materials, the sensor case, the body block and the lead wire or the lead rod and the body block have been unable to be perfectly cut off from the air, so that a slight quantity of moisture has gradually entered into the inside of the sensor case from the air and thus may badly influence a highly accurate measurement according to circumstances.
In addition, even though an inert gas (for example nitrogen gas) is enveloped in said inside of the sensor case during the production, the sensor case, the body block and the lead wire or the lead rod and the body block can not be perfectly cut off from the air, so that a slight quantity of moisture gradually enters the inside of the sensor case to reduce the accuracy of measurement and thus deteriorate the sensor.
Furthermore, usually a thin-walled capillary tube having a wall-thickness of 0.1 mm or less is used as said metallic capillary tube serving as a measuring passage, so that, in the case where a fluid flowing through said measuring passage is a highly corrosive fluid (for example HF, HCl and the like), there is also the possibility that said corrosive fluid is leaked outside from the sensor case due to a corrosion of a wall of the capillary tube.
Besides, the above described problem has the possibility of similarly occurring also in the sealing-up method of the lead wire or lead rod used as a terminal for introducing an electric current into the sensor coil.